1. Field of the Invention
The present invention relates to a homeotropic-alignment liquid crystal display device for use in a flat panel display, an optical computer, or a video projector, and a method of producing such homeotropic-alignment liquid crystal display device. More particularly, it relates to improvements in the display quality and mass production capability of the homeotropic-alignment liquid crystal display device.
2. Description of the Related Art
Homeotropic-alignment liquid crystal display devices, because of their sharp threshold in the V-T (voltage vs. transmission) characteristics and high contrast ratios, succeeded in providing high display qualities even when activated with a simple matrix driver. (SID 89 DIGEST, p378 (1989)).
In the homeotropic-alignment liquid crystal display devices, a nematic crystal liquid of negative dielectric anisotropy is used. Liquid crystal molecules homeotropically aligned with respect to substrate surfaces are caused to tilt by a drive voltage, whereupon a display operation begins. With this construction, if the molecule tilt direction is not uniform in the activated state, a remarkable brightness non-uniformity is observed. In order to avoid this phenomenon, a small pre-tilt angle is given when a liquid crystal homeotropic-alignment is achieved. However, as the pre-tilt angle increases, the difficulty in obtaining the homeotropic alignment increases. Likewise, the contrast ratio and the threshold level decrease with an increase in pre-tilt angle.
The pre-tilt angle may be given by using an angledeposited film, i.e., a film formed by an angle vapor deposition process also called as an oblique evaporation process. The angle vapor deposition process is an outstanding process used theretofore for aligning liquid crystal molecules, wherein the vapor of an oxide such as an SiO is deposited on a substrate surface from an oblique direction. However, when a small pre-tilt angle is given, if the drive voltage undergoes a sudden change, the liquid crystal molecules may tilt in diametrically opposite directions. This phenomenon is observed as a dynamic alignment defect. In order to solve this problem, a two-step angle vapor deposition process, such as disclosed in Japanese Patent Publication No. 55-13338, has been proposed as a process which is suitable for controlling the molecule tilt direction of the liquid crystal.
The above-mentioned angle vapor deposition process, however, has a problem that the pre-tilt angle varies greatly with a slight change in film thickness or incident angle. As is also reported in Proceedings of the SID, Vol. 31/4, p321 (1990), when the angle vapor deposition process is used in the production of liquid crystal light valves, a great change in the constant ratio is observed with a slight change in the pre-tilt angle. This phenomenon will bring about the occurrence of non-uniform liquid crystal alignment and an inadequate reproducibility problem. In addition, the SiO film or SiO.sub.x film is chemically unstable and hence involves an aging problem.
According to another alignment method disclosed in Japanese Patent Laid-open Publication No. 51-129251, an SiO.sub.2 film deposited on a substrate is etched off by an Ar ion beam directed obliquely onto the SiO.sub.2 film so as to produce a surface profile change which in turn is used for liquid crystal alignment. Due to a large etch-off amount, the disclosed alignment method is time-consuming.
In addition, a simple ion beam etching effected in an oblique direction is likely to cause an alignment non-uniformity problem in the activated state, which problem is particularly significant for a high resolution spatial light modulator constructed to display dynamic images.